Stereo 70 6B4G Mod - Round 7

Posted by stellavox@excite.com (A) on December 10, 2005 at 07:57:57

Guys,

I want to add some info about this mod.

Just finished bulding Serial #2 to try and establish if the great sound I was getting from S/N #1 was repeatable, and if I could use currently available tubes for it. I found the answer to be yes to both questions.

First some history; I stumbled onto this "design" 2 or three years ago when I needed an amp to drive a set of Quad 63's I had purchased. Tried a number of classic PP amps but was floored when I tried a SE 300B. Needed more power than that and had a basket of Stereo 70 parts from which this design evolved (which I believe rivals SE designs in sonics). Started with an octal input/driver based on the old Acrosound Triode circuit but eventually settled on an updated version of the Hedge differential cascode circuit after "hearing" how much the clarity improved by being able to eliminate a capacitor in the driver chain.

Based on my research,this "modern" 6DJ8 input circuit design was first used on Stereo 70 input boards available from "Tube God", one of Harvey Rosenberg's many (failed) audio excursions in the 1970's. Joe Curcio's current CCDA board is similar to that design. A schematic of his design for the Sonic Frontiers SFM-75 can be found at:

http://www.diyparadiso.com/proj/sfm75.htm

The main difference between mine and Joes is that he uses a transistor-based constant current source and I use an LM334Z set for around 5 MA. I also don't regulate any voltages (although it probably will sound better). After the tube regulator, I split off to two chokes and two following 300Ufd (or so) caps to have kind of a dual mono power supply.

Anyway, what we (myself and two other gentlemen from our Connecticut AUdio Society) were able to verify last Sunday when comparing both S/N 1 and 2 (other equipment was a modded Norh CD player, Slagle autoformer volume conteol and Quad 63's) were:

1. New Sovtek 6B4G's sound very similar to old stock Sylvanias and GE's that we had on hand. (Tube Depot sells matched pairs of Sovteks for $60}.
2. Amperex Holland 6DJ8 drivers sounded the best of what we tried (but are very expensive). Surprisingly, Phillips ECG 6922's sounded very good and are apparently readily available.
3. The cathode resistor bypass cap is not needed. This is good news because I blew up one mounted too close to the resistor which gets really hot. Sonically, removing the cap seemd to reduce volume VERY slightly but may have added a little extension to the bass. O yeah, the 4 ohm tap provides the best match to the speakers I have tried.
4. Regarding rectifiers. I have settled on the 5R4 which provides around 370 VDC to the CT of the output transformer and redults in about 60 volts across the 750 ohm cathode resistor or about 40 ma through each output tube. I tried other rectifiers including the 5AR4, which increased the B+ to close to 400V with an accompanying increase in Bias voltage/current (actally also blew up a 10 watt sand resistor - found/installed big NOS 20 watt WW's instead). 5R4 seems a nice compromise and the power transformer doesn't run too hot.
5. I currently don't use any feedback. More experimentation is necessary here - quickly tried a little bit and found that the bass does tighten up but with more than 3dB or so of feedback,the imaging starts to collapse.

You can see top and bottom pictures of S/N #2 in my inmate file. I have drawn a schematic of the amp but it is in whatever format the expressPCB schematics use.

ENJOY

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Topic - Stereo 70 6B4G Mod - Round 7 - stellavox@excite.com 07:57:57 12/10/05 (20)

the variant I built for my friend Will.... - Jim Doyle 21:00:58 12/10/05 (5)

The 845 can flab at times... - Allen Wright 03:02:41 12/12/05 (3)

Damping factor question - Naz 22:32:56 12/13/05 (2)

Look at it like this - Russ57 06:52:23 12/14/05 (1)

Yes but ... - Naz 19:49:12 12/14/05 (0)

Re: the variant I built for my friend Will.... - drlowmu 08:44:14 12/11/05 (0)

Re: Stereo 70 6B4G Mod - Round 7 - zacster 15:39:08 12/10/05 (0)

Re: Stereo 70 6B4G Mod - Round 7 - Allen Wright 12:35:52 12/10/05 (4)

Re: Stereo 70 6B4G Mod - Jfet based CCS - stellavox 13:25:18 12/10/05 (3)

Re: Stereo 70 6B4G Mod - Jfet based CCS - Allen Wright 03:15:01 12/12/05 (2)

Re: Stereo 70 6B4G Mod - Jfet based CCS - THANKS - stellavox 09:30:11 12/12/05 (1)

Let us know if it improves - OK? (nt) - Allen Wright 13:26:39 12/12/05 (0)

Re: Stereo 70 6B4G Mod - Round 7 - fatbottle 11:31:50 12/10/05 (2)

Re: Stereo 70 6B4G Mod - headroom? - stellavox 13:30:29 12/10/05 (1)

Re: Stereo 70 6B4G Mod - 150 Vp-p from driver - stellavox 05:18:23 12/11/05 (0)

Re: Stereo 70 6B4G Mod - Round 7 - Eli Duttman 11:14:42 12/10/05 (2)

Re: Stereo 70 6B4G Mod - JJ E88CC - stellavox 13:33:45 12/10/05 (1)

that 6NP1's that I just got... - Allen Wright 03:17:27 12/12/05 (0)

Nice work by the looks of things, where is the... - wpod 10:29:33 12/10/05 (1)

Re: where is schematic? - stellavox 13:36:03 12/10/05 (0)

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